1. Field of the Invention
The present invention relates to a charging apparatus for charging a secondary battery.
2. Description of the Related Art
In a charging apparatus which is used for charging a secondary battery such as a lithium battery or the like, a constant current charge and a constant voltage charge are generally executed.
The charging apparatus using such a charging method has a construction, for example, as shown in FIG. 10. FIG. 11 shows a relation (output characteristics of the charging apparatus) between a charge voltage V and a charge current I. FIG. 12 shows relations (charge characteristics curves) between the charge voltage V, charge current I and a charge time T.
In a charging apparatus 100, a secondary battery 101 and a current detecting resistor 102 are serially connected to a power source 103. That is, a positive terminal of the secondary battery 101 is connected to a positive terminal of the power source 103. A negative terminal of the secondary battery 101 is connected to one end of the current detecting resistor 102 and the other end of the current detecting resistor 102 is connected to a negative terminal of the power source 103. Positive and negative input terminals of a comparator 104 are connected to both ends of the current detecting resistor 102 and an output terminal of the comparator 104 is connected to the power source 103.
In such a construction, since a charge of a constant current Ib1 is finished and a charge of a constant voltage Vb1 is started at the end of the charge of the secondary battery 101, the charge current I is reduced. When the comparator 104 detects that the charge current I is equal to or less than a predetermined value E1, a generation of a charge continuation signal SE to the power source 103 is stopped, thereby finishing the charge. That is, the comparator 104 detects the voltage across the current detecting resistor 102, thereby detecting that the secondary battery 101 is fully charged.
As a foregoing current detecting resistor 102 of the charging apparatus 100, a resistor of a low resistance, for example, (R=0.1.OMEGA.) is used because of a reason of an electric power consumption or the like. However, since the charge current I at the end of the charge of the secondary battery 101 is a low current, for example, I=0.2A, a voltage which is detected by the comparator 104 is an extremely microscopic voltage as E1=I.multidot.R=20 mV. As a comparator 104 for detecting such a microscopic voltage, a high-precision comparator in which an offset voltage is extremely low has to be used, so that there is a drawback that it is expensive.
Even after completion of the charge, there is a case such that a capacity of the battery is reduced by a consumption in the battery, a consumption due to an impedance of the power source 103 when the apparatus is left in a stop state, a consumption, for example, in a portable telephone main body, or the like. Generally, when the charge is continued even after the end of the charge, the occurrence of the drop in the battery capacity can be also prevented. However, there is a problem such that when a voltage is continuously applied to the secondary battery 100, the life of the battery is shortened.
Further, when a plurality of, for example, two secondary batteries are connected in parallel and are charged by using the foregoing charging apparatus 100, if during the charge of one of the secondary batteries, the other secondary battery is connected, there is a problem such that there is a case where the charge is finished by an erroneous discrimination irrespective of a fact that one of the secondary batteries is being charged.